Vibrational spectra of p-carboxylate and p-sulfonate azocalix[4]arene

Abstract

The vibrational spectra of calixarenes with azobenzene units having sulfonate (p-SAC) and carboxylate (p-CAC) groups in the para position were recorded and analyzed. The optimization of the structure and analysis of normal vibration for calixarenes were performed using the DFT method. The calculated geometric parameters, the frequencies of the harmonic oscillations, the band intensities in the IR spectra, and the Raman scattering activity of the calixarenes are consistent with the experimental data. The p-CAC and p-SAC calix[4]arene molecules are in conical conformation due to the ring system of H-bonds along the low rim. The H-bond is weaker in the p-SAC molecule. The orientation of the aromatic moieties depends on the type of terminal functional groups. The energy differences between the E-and Z-forms of azobenzene groups in p-CAC and p-SAC are 60.8 and 62.6 kcal/mol, respectively. The experimental vibrational spectra of the calixarenes were interpreted using the potential energy distribution. Bands characteristic of trans and cis conformations of azobenzene fragments have been assigned. The HOMO and LUMO frontal molecular orbitals of the calixarene molecules are localized. The studied calixarenes can be used as antioxidants for thermal and light stabilization of polymer building materials.